1. Field of the Invention
The present invention relates to a digital video tape recorder (VTR) and a record carrier therefore, and more particularly, to a digital VTR and record carrier capable of forming/providing a video image during a picture search (so-called high-speed search or trickplay) mode where the record carrier is moved past the VTR playback heads at a speed different from the recording speed.
2. Description of the Prior Art
In a VTR, magnetic pick-up/record heads are mounted on a cylindrical drum, and a record carrier (e.g., magnetic tape) is wrapped about one-half way around the drum in a helical manner. As shown in FIG. 1, during recording or playback, the drum (not shown) rotates and the tape 10 is driven in the direction indicated by arrow 12 past the drum at a constant speed, thereby recording or picking-up video information signals in parallel slant (helical) tracks 14. An example of the, movement of the heads relative to tracks 14 is shown by arrow 16.
During high-speed playback (or so-called trickplay) the tape is driven past the drum at a speed higher than the recording speed. This results in the magnetic heads sweeping across multiple slant tracks for each scan across the tape, as shown by the dotted arrow in FIG. 1, picking-up only a portion of the video information signal from each recorded track.
A digital VTR providing a high-speed playback mode presents some troublesome requirements. This is so because conventional digital image compression coding techniques typically use a variable number of bits to code an image, depending upon the image complexity. Therefore, when the VTR is operated in the high-speed playback mode, continuous recovery of the coded data is not possible and only portions of the recorded image are recovered. Without complete recovery of all of the coded data, image reconstruction will be severely degraded.
In order to allow proper operation of the high-speed search mode in a digital VTR, there are two basic requirements. First, there must be a fixed relationship between the position on the tape of the recovered high-speed data and the position of the reconstructed image that this data represents. Second, the bit rate of the recorded data must be fixed for some sequence of the recorded images, so that linear advances on the tape correspond to equivalent advances of the image sequence. These requirements conspire to make the application of variable length coding schemes difficult to use for digital VTR's. Since many of the current low bit rate image compression coding algorithms use a variable bit length coding schemes (incorporating, e.g., interframe motion compensation), they are not well-suited for application to digital VTRs.
U.S. Pat. No. 5,136,394 issued Aug. 4, 1992 to Haikawa et al. discloses a digital VTR having an image coding scheme which accommodates a picture search mode of operation. As disclosed therein, each image is divided into three types of pixels, namely, a, b and c, respectively, where there are twice as many c type pixels per image line as there are a and b type pixels per image line. The a, b and c type of pixels for each image are grouped together and then sequentially recorded on the tape. Furthermore, each of the groups are sub-divided into two further sub-groups, corresponding to the upper m bits and the lower n bits of the digital data representative of each pixel. During a picture search mode, only the upper bits of e.g., the "a" type pixels are recovered from the tape for providing a usable "rough" image. However, very limited picture search freedom is provided by this technique, since scanning and recovery of the pre-recorded "a" type pixels on the tape must occur.
U.S. Pat. No. 5,136,391 issued Aug. 4, 1992 to M. Minami describes a digital VTR wherein the input image is successively subsampled to divide it into a main image having low resolution components and two hierarchical subimages having the higher resolution image components. The main image is fixed bit-length coded and recorded along a central portion of each magnetic tape track and the subimages are variable bit-length coded (using, for example, adaptive DCT techniques) and recorded on the magnetic tape tracks symmetrically about opposite sides where the main image is recorded. This technique places undesirable requirements on the trickplay operation and apparatus, since it is required that the central portion of each previously recorded track be recovered in order to reconstruct a usable image.
Another technique for solving this problem is described in U.S. Pat. No. 4,807,053 issued Feb. 21, 1989 to Mr. Heignemas. As described therein, an image compression algorithm is used which results in a poor reconstruction of the image when less than all of the recorded picture data is recovered, and, when all of the data is recovered results in the best reconstruction of the image. First, the input image is divided into sub-image blocks which are encoded using a first transform coding technique. Then, successive subimages are analyzed for motion and given a motion code depending on the degree of difference between the motion of the sub-images. If the next sub-image represents little motion from the prior sub-image, as indicated by it's motion code, a second transform coding technique could be used, which, when combined with the prior sub-image, enables more accurate reconstruction of the input sub-images. If, however, the motion code indicates that there is greater than a certain minimum amount of motion between the successive sub-images, then the first encoding technique is used again. Thus, when all the sub-images are recovered in order, during normal play, the use of the motion codes allows sub-images with similar motion codes to be combined (even those from prior frames or fields, thus equivalent to an interframe coding type of processing) which is an advantageous image compression technique. However, if less than all the sub-image data is recovered, only those sub-images having similar motion codes and specific transform codes can be combined. If the successively recovered sub-images don't have motion codes and transform codes which allow their combination for generating a higher accuracy sub-image, they are not combined and instead the prior recovered sub-image is repeated for reconstructing the original image. This technique is not particularly advantageous since it results in a "blockiness" in the recovered signal and furthermore, the data compression is not as effective as other of the more conventional types of image compression algorithms.
Another technique developed for video image compression which takes into account the "sample skipping" inherent when recovering data from a record carrier at a speed other than its recording speed, is described by Wu et al. in an article entitled "Rate-Constrained Optimal Block-Adaptive Coding for Digital Tape Recording of HDTV" published in the IEEE Transactions on Circuits and Systems for Video Technology, Vol. 1, No. 1, March 1991. In this technique, each frame (or field) of video is partitioned into a small number of sub-images. Each sub-image is partitioned into non-overlapping blocks and each block is coded by one of a finite set of predesigned block quantizers covering a range of bit rates, which results in each sub-image being independently coded with a fixed number of bits. A near-optimal quantizer allocation algorithm based on the Lagrange Multiplier method is used to select a particular quantizer for each block. The objective is to minimize the distortion of the entire sub-image under the constraint of a fixed number of total bits for each sub-image. This rate-constrained block-adaptive technique utilizes a multi-stage compression algorithm comprising discrete cosine transformation followed by vector quantization. Although this technique allows for data reconstruction during trickplay, the multi-stage compression coding technique is expected to result in an undesirable "blockiness" of the reconstructed image during trickplay. Furthermore, the block-adaptive technique results in less than optimum data compression.